Today, detection and quantification of a tumor marker, a specific protein or other antigen contained in blood or urine or in other biological sample of human or animal are widely performed for diagnosis in the medical field as well as for research in the fields of biology and biochemistry. As a method of specifically detecting a trace amount of a compound to be measured (antigen), immunoassay is employed. One example thereof is a sandwich method in which an antigen captured by a primary antibody is labeled with a secondary antibody, and a fluorescent antibody method which uses a fluorescent label as a secondary antibody and radioimmunoas say which uses labeling with a radioactive substance have been widely employed.
Meanwhile, lectins are known as proteins binding to a sugar chain that are not produced by an immunological method. Lectins usually have two or more sugar-binding sites per molecule. Generally, proteins and the like that are contained in a biological sample comprise a sugar chain, and there has also been proposed a method in which a lectin that specifically recognizes a sugar chain of an antigen to be measured in place of a secondary antibody in an immunoassay method.
However, in biological samples such as blood (serum and plasma), in addition to an antigen to be measured, a variety of proteins, lipids and other impurities are contained. In immunoassay, due to non-specific adsorption of these impurities to, for example, a primary or secondary antibody or a solid-phase layer (solid-phase support) by which a primary antibody is immobilized to a support (such as a well) and binding of a fluorescent label to the impurities, non-specific signals originating from the impurities are generated to cause background noise.
As an immunoassay method in which non-specific reactions are suppressed, the use of a sugar chain compound having a non-specific reaction-reducing effect has been examined in an immunoassay method using a measurement reagent having a sugar chain (Patent Document 1). In this case, it is believed that the sugar chain compound acts to reduce noise by allowing the added other sugar chain compound to block a reaction site of the measurement reagent having a sugar chain.
In this manner, there have already been made proposals pertaining to noise reduction in the measurement of an antigen in a biological sample by immunoassay; however, from the standpoints of the measurement sensitivity, measurement accuracy, operability, cost and the like, it is critical to select the most effective method in accordance with each biological sample and antigen to be measured, and there is a demand for a proposal pertaining to further noise reduction.